1) Field of the Invention
This invention pertains to the field of digital communication systems, and more particularly to digital communication signal detection and synchronization.
2) Background of the Related Art
Many modern digital communications systems utilize burst type transmissions in which a relatively small number of information bits are transmitted by sending a sequence of a small number of waveforms, say “N” in number, termed “symbols,” each of which symbols can assume one of “Q” possible shapes. As an example, Q=4 for the case of quadraphase shift keying (QPSK).
In a burst-type communication system it is important to rapidly detect the onset of the signal at the receiver and to perform time synchronization to it. To facilitate this, in many burst-type communication systems a burst consists of a initial set of symbols, termed a “preamble,” which permits rapid synchronization, and a subsequent set of symbols, termed a “payload,” which represents the actual message data. Some systems also include special data in the middle of the burst, or at the end, which is termed a “postamble.”
Time synchronization for a burst-type communication system includes two aspects: (1) determining the timing of the symbols, and (2) determining the onset of the actual payload data carried by the burst. It is important that such synchronization be performed using the preamble data so that the payload data may be correctly decoded. Determining the onset of payload data in a burst transmission is typically performed through the transmission during the preamble of a specific sequence of symbols, termed a “unique word.”
Time synchronization is to be distinguished from determination of received signal carrier frequency and phase, which is termed “carrier synchronization.”
In addition to allowing time synchronization, the preamble may be used to transmit a limited amount of information that describes the subsequent payload data. For example, such information may specify the length of the payload (number of symbols), the modulation format of the payload data, or other information.
In conventional systems the above information is sent in the preamble by the normal manner of associating individual symbols with information bits. However, in situations in which the received signal level is low, such individual symbols may be obscured by noise such that this conventional approach will lead to very high error rates.
Accordingly, it would be advantageous to provide a system and method which will perform both time synchronization and information transmission via a preamble when the input signal level is low. Other and further objects and advantages will appear hereinafter.